
def check(N, c):
    RinRight = c.count('R')
    WinLeft = 0
    r = RinRight
    for i in range(N):
        if c[i] == 'R':
            RinRight -= 1
        else: # c[i] == 'W'
            WinLeft +=1
        r = min(r, max(RinRight,WinLeft))

    return r


N = int(input())
c= input()
print(check(N,c)) def check(N, c):
    RinRight = c.count('R')
    WinLeft = 0
    r = RinRight
    for i in range(N):
        if c[i] == 'R':
            RinRight -= 1
        else: # c[i] == 'W'
            WinLeft +=1
        r = min(r, max(RinRight,WinLeft))

    return r check RinRight = c.count('R') RinRight c.count('R') c.count c count 'R' WinLeft = 0 WinLeft 0 r = RinRight r RinRight for i in range(N):
        if c[i] == 'R':
            RinRight -= 1
        else: # c[i] == 'W'
            WinLeft +=1
        r = min(r, max(RinRight,WinLeft))

     i range(N) range N if c[i] == 'R':
            RinRight -= 1
        else: # c[i] == 'W'
            WinLeft +=1
         c[i] == 'R' c[i] c i 'R' RinRight -= 1 RinRight 1 WinLeft +=1 WinLeft 1 r = min(r, max(RinRight,WinLeft)) r min(r, max(RinRight,WinLeft)) min r max(RinRight,WinLeft) max RinRight WinLeft return r r N N c c N = int(input()) N int(input()) int input() input c= input() c input() input print(check(N,c)) print check(N,c) check N c def check(N, c):
    RinRight = c.count('R')
    WinLeft = 0
    r = RinRight
    for i in range(N):
        if c[i] == 'R':
            RinRight -= 1
        else: # c[i] == 'W'
            WinLeft +=1
        r = min(r, max(RinRight,WinLeft))

    return r def check(N, c):
    RinRight = c.count('R')
    WinLeft = 0
    r = RinRight
    for i in range(N):
        if c[i] == 'R':
            RinRight -= 1
        else: # c[i] == 'W'
            WinLeft +=1
        r = min(r, max(RinRight,WinLeft))

    return r check N = int(input()) int(input()) N c= input() input() c

def check(N, c):
    RinRight = c.count('R')
    WinLeft = 0
    r = RinRight
    for i in range(N):
        if c[i] == 'R':
            RinRight -= 1
        else: # c[i] == 'W'
            WinLeft +=1
        r = min(r, max(RinRight,WinLeft))

    return r


N = int(input())
c= input()
print(check(N,c))